Textile machines of this type are well known. Conventionally, in many false twist texturing machines, the heating of the yarns is performed by passing the yarns in contact with the surface of a heated plate. Parallel grooves are formed in the surface of the plate so as to guide the yarns and prevent interference of one yarn with an adjacent yarn. Such a plate is formed having a curvature of relatively large radius in the longitudinal direction of the passage of the yarns over the surface, so that the tension in the yarns keeps the yarns in contact with that surface and maintains control of the long lengths of yarn. This arrangement facilitates the transfer of heat from the heated surface to the yarns, thereby keeping to a minimum the length of the heater required to raise the temperature of the yarns to that desired for correct processing. For heaters of between 0.2 m and 3 m in length, typical curvatures lie in the range of 5 m to 20 m radius.
It is an obvious aim of textured yarn producers to maximise the production of textured yarn from any texturing machine, by increasing the machine speed and hence the throughput speed of the yarn. It is also desirable to minimise the length of the heater by maximising the rate of heat transfer from the heater to the yarn. One of the limitations to increasing the speed of the machine is the `surge speed`, a yarn throughput speed at which dynamic threadline instability occurs. This speed is affected by yarn tension, the rate of twist insertion and machine configuration. At this speed, the high rotational speed of the twisting yarn tends to create uncontrolled vibrations in the running yarn, and this causes rapid variations in tension and in the twist level inserted in the yarn by the twisting unit, thereby producing unacceptable yarn. It has always been regarded as essential that the vibrations in the yarn be minimised throughout the heating and cooling zones so as to raise the surge speed as much as possible. To this end, and to maximise the heat transfer to the yarn, the yarn is controlled by being maintained in contact with the heater surface by virtue of the yarn tension and the longitudinal curvature of the heater. Alternatively, for high temperature and non-contact heaters, guides have been located on the heater to provide a curved or sinuous path for the yarn on or adjacent the heated surface.